EMSL: M Lizabeth Alexander's Publications

de Gouw JA, D Welsh-Bon, C Warneke, WC Kuster, ML Alexander, AK Baker, AJ Beyersdorf, DR Blake, MR Canagaratna, AT Celada, LG Huey, W Junkermann, TB Onasch, A Salcido, SJ Sjostedt, A Sullivan, DJ Tanner, L Vargas-Ortiz, RJ Weber, DR Worsnop, XY Yu, and RA Zaveri. 2009. "Emission and Chemistry of Organic Carbon in the Gas and Aerosol Phase at a Sub-Urban Site Near Mexico City in March 2006 During the MILAGRO Study." Atmospheric Chemistry and Physics 9(10):3425-3442. Abstract Volatile organic compounds (VOCs) and carbonaceous aerosol were measured at a sub-urban site near Mexico City in March of 2006 during the MILAGRO study (Megacity Initiative: Local and Global Research Objectives). Diurnal variations of hydrocarbons, elemental carbon (EC) and hydrocarbon-like organic aerosol (HOA) were dominated by a high peak in the early morning when local emissions accumulated in a shallow boundary layer, and a minimum in the afternoon when the emissions were diluted in a significantly expanded boundary layer and, in case of the reactive gases, removed by OH. In comparison, diurnal variations of species with secondary sources such as the aldehydes, ketones, oxygenated organic aerosol (OOA) and water-soluble organic carbon (WSOC) stayed relatively high in the afternoon indicating strong photochemical formation. Emission ratios of many hydrocarbon species relative to CO were higher in Mexico City than in the U.S., but we found similar emission ratios for most oxygenated VOCs and organic aerosol. Secondary formation of acetone may be more efficient in Mexico City than in the U.S., due to higher emissions of alkane precursors from the use of liquefied petroleum gas. Secondary formation of organic aerosol was similar between Mexico City and the U.S. Combining the data for all measured gas and aerosol species, we describe the budget of total observed organic carbon (TOOC), and find that the enhancement ratio of TOOC relative to CO is conserved between the early morning and mid afternoon despite large compositional changes. Finally, the influence of biomass burning is investigated using the measurements of acetonitrile, which was found to correlate with levoglucosan in the particle phase. Diurnal variations of acetonitrile indicate a contribution from local burning sources. Scatter plots of acetonitrile versus CO suggest that the contribution of biomass burning to the enhancement of most gas and aerosol species was not dominant and perhaps not dissimilar from observations in the U.S.

Fast JD, A Aiken, JD Allan, ML Alexander, T Campos, MR Canagaratna, EG Chapman, P DeCarlo, B de Foy, J Gaffney, JA de Gouw, JC Doran, L Emmons, A Hodzic, SC Herndon, LG Huey, JT Jayne, JL Jimenez, LI Kleinman, WC Kuster, NA Marley, LM Russell, C Ochoa, TB Onasch, MS Pekour, C Song, IM Ulbrich, C Warneke, D Welsh-Bon, C Wiedinmyer, DR Worsnop, XY Yu, and RA Zaveri. 2009. "Evaluating Simulated Primary Anthropogenic and Biomass Burning Organic Aerosols during MILAGRO: Implications for Assessing Treatments of Secondary Organic Aerosols." Atmospheric Chemistry and Physics 9(16):6191-6215. Abstract Simulated primary organic aerosols (POA), as well as other particulates and trace gases, in the vicinity of Mexico City are evaluated using measurements collected during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaigns. Since the emission inventories and dilution will affect predictions of total organic matter and consequently total particulate matter, our objective is to assess the uncertainties in predicted POA before testing and evaluating the performance of secondary organic aerosol (SOA) treatments. Carbon monoxide (CO) is well simulated on most days both over the city and downwind, indicating that transport and mixing processes were usually consistent with the meteorological conditions observed during MILAGRO. Predicted and observed elemental carbon (EC) in the city was similar, but larger errors occurred at remote locations since the CO/EC emission ratios in the national emission inventory were lower than in the metropolitan emission inventory. Components of organic aerosols derived from Positive Matrix Factorization and data from several Aerodyne Aerosol Mass Spectrometer instruments deployed both at ground sites and on research aircraft are used to evaluate the model. Predicted POA was consistently lower than the measured organic matter at the ground sites, which is consistent with the expectation that SOA should be a large fraction of the total organic matter mass. A much better agreement was found when predicted POA was compared with the sum of "primary anthropogenic" and "primary biomass burning" components on days with relatively low biomass burning, suggesting that the overall magnitude of primary organic particulates released was reasonable. The predicted POA was greater than the total observed organic matter when the aircraft flew directly downwind of large fires, suggesting that biomass burning emission estimates from some large fires may be too high. Predicted total observed organic carbon (TOOC) was also analyzed to assess how emission inventory estimates of volatile organic compounds may impact predictions of SOA.

Kleinman LI, SR Springston, JX Wang, PH Daum, YN Lee, LJ Nunnermacker, G Senum, J Weinstein-Lloyd, ML Alexander, JM Hubbe, JV Ortega, RA Zaveri, MR Canagaratna, and JT Jayne. 2009. "The Time Evolution of Aerosol Size Distribution Over the Mexico City Plateau." Atmospheric Chemistry and Physics 9(13):4261-4278. Abstract As part of the MILAGRO field campaign, the DOE G-1 aircraft was used to make measurements over and downwind of Mexico City with the objective of determining growth characteristics of aerosols from a megacity urban source. This study focuses on number concentration and size distributions. It is found that a 5-fold increase in aerosol volume is accompanied by about a 5-fold increase in accumulation mode number concentration. There is growth in aerosol volume because there are more accumulation mode particles, not because particles are larger. Condensation and volume growth laws were examined to see whether either is consistent with observations. Condensation calculations show that the growth of Aitken mode particles into the accumulation mode size range gives the required increase in number concentration. There are minimal changes in the accumulation mode size distribution with age, consistent with observations. Volume-growth in contrast yields a population of large particles, distinctly different from what is observed. Detailed model calculations are required to translate our observations into specific information on the volatility and properties of secondary organic aerosol.

Spicer CW, MW Holdren, KA Cowen, DW Joseph, JR Satola, BP Goodwin, H Mayfield, A Laskin, ML Alexander, JV Ortega, MK Newburn, RH Kagann, and RA Hashmonay. 2009. "Rapid Measurement of Emissions from Military Aircraft Turbine Engines by Downstream Extractive Sampling of Aircraft on the Ground: Results for C-130 and F-15 Aircraft." Atmospheric Environment 43(16):2612-2622. doi:10.1016/j.atmosenv.2009.02.012 Abstract Aircraft emissions affect air quality on scales from local to global. About 10% of the aviation fuel used in the U.S. is consumed by military aircraft, and emissions from this source are facing increasingly stringent environmental regulations, so improved methods for quickly and accurately determining emissions from existing and new engines are needed. This paper reports results of a study to advance the methods used for detailed characterization of military aircraft emissions, and provides emission factors for two aircraft; the F-15 fighter and the C-130 cargo plane. The new approach employs a strategy of outdoor ground-level sampling downstream behind operational military aircraft. This permits rapid change-out of the aircraft so that engines can be tested quickly on operational aircraft Measurements were made at throttle settings from idle to afterburner using a simple extractive probe in the dilute exhaust. Emission factors determined using this approach agree very well with those from the traditional method of extractive sampling at the exhaust exit. Emission factors are reported for CO2, CO, NO, NOx, and more than 60 hazardous and/or reactive organic gases. Intra-engine and engine to engine variability were assessed. For both engines, the effect of engine power on emissions was as expected, with higher power leading to reduced emission factors for CO and organic gases and higher emission factors for nitrogen oxides. At afterburner power, the F-15 engine yielded higher emission factors for CO and many organics and lower NOx emission factors compared with the military power throttle setting. The C-130 turboprop engine generally produced higher CO andorganic emissions and lower NOx emissions per unit of fuel consumed than the F-15 engines. Comparison of the emissions of nine hazardous air pollutants from these two engines with emissions from nine other aircraft engines also is discussed.

Yu Y, ML Alexander, V Perraud, E Bruns, S Johnson, M Ezell, and BJ Finlayson-Pitts. 2009. "Contamination from electrically conductive silicone tubing during aerosol chemical analysis." Atmospheric Environment 43(17):2836-2839. Abstract Electrically conductive silicone tubing is used to minimize losses in sampling lines during the analysis of airborne particle size distributions and number concentrations. We report contamination from this tubing using gas chromatography-mass spectrometry (GC-MS) of filter-collected samples as well as by particle mass spectrometry. Comparison of electrically conductive silicone and stainless steel tubing showed elevated siloxanes only for the silicone tubing. The extent of contamination increased with length of tubing to which the sample was exposed, and decreased with increasing relative humidity.

Hu J, RA Dagle, BR Johnson, HW Kreuzer, DJ Gaspar, BQ Roberts, and ML Alexander. 2008. "Development of a Micropyrolyzer for Enhanced Isotope Ratio Measurement." Industrial and Engineering Chemistry Research 47(22):8625-8630. doi:10.1021/ie8009236 Abstract This paper presents design, fabrication and testing of a micro scale reactor for the pyrolysis of organic compounds. The reactor system described here is suitable for use in enhanced isotope ratio measurement in a continuous flow mode. A characteristic of such a system is it can be utilized to pyrolyze organic compounds with sample size 20-50 times smaller than conventional. Results have shown that organic compounds, such as 1-butanol, ethanol, and ethanol amine, can be fully decomposed to desired products CO and H2, at temperature of 1200oC, which is 200oC lower than conventionally reported. Undesired products methane and CO2 are eliminated in the pyrolysis process. The proof-of-concept experimental results clearly demonstrate that the micro pyrolyzer can be readily integrated with isotope ratio mass spectrometer (IRMS) to differentiate between different sources of the same materials.

Kleinman LI, SR Springston, PH Daum, YN Lee, LJ Nunnermacker, G Senum, JX Wang, J Weinstein-Lloyd, ML Alexander, JM Hubbe, JV Ortega, MR Canagaratna, and J Jayne. 2008. "The Time Evolution of Aerosol Composition Over the Mexico City Plateau." Atmospheric Chemistry and Physics 8(6):1559-1575. Abstract The time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILAGRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH3CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as −Log10 (NOx/NOy). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NOx/NOy from nearly 100% to 10%.

Wang JX, YN Lee, PH Daum, JT Jayne, and ML Alexander. 2008. "Effects of aerosol organics on cloud condensation nucleus (CCN) concentration and first indirect aerosol effect." Atmospheric Chemistry and Physics 8(21):6325-6339. Abstract Abstract. Aerosol microphysics, chemical composition, and CCN properties were measured on the Department of Energy Gulfstream-1 aircraft during the Marine Stratus/ Stratocumulus Experiment (MASE) conducted over the coastal waters between Point Reyes National Seashore and Monterey Bay, California, in July 2005. Aerosols measured during MASE included free tropospheric aerosols, marine boundary layer aerosols, and aerosols with high organic concentration within a thin layer above the cloud. Closure analysis was carried out for all three types of aerosols by comparing the measured CCN concentrations at 0.2% supersaturation to those predicted based on size distribution and chemical composition using K¨ohler theory. The effect of aerosol organic species on predicted CCN concentration was examined using a single hygroscopicity parameterization.

Yu Y, MJ Ezell, A Zelenyuk, DG Imre, ML Alexander, JV Ortega, JL Thomas, K Gogna, DJ Tobias, B D'Anna, CW Harmon, S Johnson, and BJ Finlayson-Pitts. 2008. "Nitrate Ion Photochemistry at Interfaces: A New Mechanism for Oxidation of alpha-Pinene." Physical Chemistry Chemical Physics. PCCP 10(21):3063-3071. doi:10.1039/b719495a Abstract The photooxidation of 0.6 - 0.9 ppm α-pinene in the presence of a deliquesced thin film of NaNO3, and for comparison increasing concentrations of NO2, was studied in a 100 L Teflon® chamber at relative humidities from 70 − 88% and temperatures from 296 − 304 K. The loss of α-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO3 experiments than in NO2 experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO3 photolysis. These data indicate enhanced loss of α-pinene at the NaNO3 thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that α-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics.

Yu Y, MJ Ezell, A Zelenyuk, DG Imre, ML Alexander, JV Ortega, B D'Anna, CW Harmon, S Johnson, and BJ Finlayson-Pitts. 2008. "Photooxidation of Alpha-Pinene at High Relative Humidity in the Presence of Increasing Concentrations of NOx." Atmospheric Environment 42(20):5044-5060. doi:10.1016/j.atmosenv.2008.02.026 Abstract The photooxidation of ~1 ppm alpha-pinene in the presence of increasing concentrations of NO2 was studied in a Teflon chamber at relative humidities from 70 - 88% and temperatures from 296 - 304 K. The loss of alpha-pinene and formation of gas phase products were followed using proton transfer reaction mass spectrometry (PTR-MS). Gas phase reaction products measured by PTR-MS and their yields include formaldehyde (5 + 1%), formic acid (2.5 + 1.4%), methanol (0.6 + 0.3%), acetaldehyde (3.9 + 1.7%), acetic acid (10 + 2%), acetone (11.5 + 3.1%), pinonaldehyde (22 + 6%), and pinene oxide (0.9 + 0.1%). There was evidence of organic nitrates in the gas phase and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition was probed using single particle mass spectrometry (SPLAT II). SPLAT II analysis also provided measurements of the vacuum aerodynamic diameters of the newly formed secondary organic aerosol (SOA) particles and, in combination with the electrical mobility diameter, a particle density of 1.21 + 0.02 g cm-3 was calculated, 20% larger than often assumed in calculating SOA yields. SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and organics, possibly including pinonic acid, pinic acid and trans-sobrerol. Three-wavelength light scattering measurements made using an integrating nephelometer were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon.